Brake Performance Declines Too Soon
Page 52
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A Detailed Analysis of the Many Problems which have a Bearing on Braking Efficiency : Used Commercial Vehicles Fail to Reach Necessary Deceleration Standards ACCORDING to the findings of the Road Research Laboratory contained in a paper "The Braking Performance of Motor Vehicles and Brake Testing" (Stationery Office, 25.), about 13 per cent, of vehicles in use, although possibly good when new, at present cannot stop within 75 ft. from 30 m.p.h., corresponding to 34.5 ft. from 20 m.p.h., which, it is suggested, should be accepted as a minimum standard. In tests on commercial vehicles, staged on the road, the average stopping distance from 20 m.p.h. was 46 ft. and the overall deceleration of eight-wheelers and others pulling trailers corresponded to 67 ft.
The distance in which a vehicle can be braked is stated to depend on the design and construction of the braking system and its condition and adjustment, the degree of skill shown by the driver, on his physical limitations and on the frictional properties of the tyres and road surface. To stop a vehicle in the shortest possible distance the brakes must keep all the wheels on the point of locking during the whole time required to stop, but this ideal can seldom be realized because of load transference.
Load Transference When the brakes are applied, the braking force does not act through the centre of gravity of the vehicle, resulting in the effective load on the front wheels being increased and that on the rear wheels decreased. The magnitude of load transference depends on the deceleration produced by braking on the total weight of the vehicle and the ratio of the wheelbase to the height of the centre of gravity of the vehicle plus load.
In many vehicles this ratio is 4; 1 and the static load on each axle is the same. Under such a condition, deceleration of lg. would transfer to the front wheels about half the static load on the rear wheels.
If the rear wheels are locked while the front wheels are still rolling, the vehicle tends to slew round, but the extent of deviation can be controlled by steering if the driver is skilful. If, on the other hand, the front wheels lock and the rear wheels are still rolling, the vehicle tends to continue in its original direction, but does not answer to the steering.
Discomfort or the likelihood of injury to the driver and passengers usually imposes a limit on the deceleration used. Thp maximum deceleration for .public-service vehicles deemed coin B 8
patible with the safety of passengers is 0.5g., but experiments made by the laboratory show that where the rate is greater than 0.25g., standing passengers in the bus have difficulty in retaining their balance.
Observations show that drivers differ both in the maximum effort they can apply to the brake pedal and the time taken to reach this maximum. They also differ in their reaction time, that is, time taken from their perceiving an emergency to their applying the brake.
Locked Wheels Experiments show that the friction between the tyres and the road surface is generally rather less when the wheels are locked and sliding than when they are still turning but on the point of locking. Although it is not practicable for the wheels to be brought simultaneously to a point of impending skidding, the quickest stop is usually attained in practice when all wheels are locked as rapidly as possible.
If stones in the road surface have sharp edges, high pressures are set up between these edges and the rubber of the tyre, and the water film is easily ruptured. When the edges become worn, the maintenance of satisfactory resistance to skidding is helped by providing draining channels in the road surface or by giving it a cross-grained texture, or by making draining channels in the tyre through its tread pattern. The laboratory tests have shown that, provided the tread pattern ensures that the contact area is well broken, there is little to chose between different tread patterns as regards resistance to skidding.
It is maintained that the length of skid marks does not give any reliable indication of the distance required to stop a vehicle, because the point at which they begin may depend on uncertain factors, such as the thermal properties of the tread rubber, road surface, surface temperatures of the tyre and road, and the rapidity with which the wheels can be locked.
The solenoid-operated Webley pistol, of a type similar to that used in The Commercial Motor tests, and the fifth wheel with marking attachment, are advantageous items of test equipment because they measure braking distance directly, including the distance travelled during time delay before the brakes begin to act.
Nevertheless, with this equipment it is difficult to introduce a pressure gauge to indicate pedal effort. Further, the rate of application of the brakes can influence the result and no indication is given of the degree of braking on the separate wheels.
Similar criticism is made" of other mobile equipment. The heavier and more expensive types, as used by brakegear manufacturers and the larger service stations, afford a history of braking at each wheel, but do not measure the time delay of the braking system, or register the effect of load transference.
Tests made on commercial vehicles with well-adjusted brakes indicate that the lighter commercial vehicles have a brake performance similar to that of cars, except that when fully laden, the retardation is somewhat lower. In many cases the ratio of braking distances, unladen and laden, is approximately proportional to the ratio of unladen to laden weights.
The best braking performance of a few of the heaviest commercial vehicles is said to be much lower than that of other classes, because of the slow buildup of pressure in the system. Although high maximum deceleration was achieved, the time taken to reach full braking power was, however, comparatively long.
Measurements of the braking sperformance of different makes of single and double-deck bus, unladen and when carrying the equivalent of 35 passengers, showed small differences in their retardation figures.
Vehicles selected at random on the road and subjected to brake test under police supervision showed marked variation in efficiency. Separate tests made by the police, with equipment recording maximum deceleration, were compared with braking performance measured in terms of stopping distance. A reasonable approximation to the overall deceleration could be deduced by multiplying the maximum deceleration of commercial vehicles by a factor varying from 0.75-0.50.
Average Value 0.56g.
Of the commercial vehicles tested, the average value of maximum deceleration was 0.56 g. (24 ft. from 20 -m.p.h.); 14 per cent. did not attain 0.4 g. (33.5 ft. from 20 m.p.h.) and 5 per cent. 0.3 g. (44.7 ft. from 20 m.p.h.). Particulars of the commercial vehicles stopped for these tests did not include precise information of the state of loading, and further trials were made on the A45 road, in which the vehicles were weighed and the braking distances measured, using recording decelerometers.
The mean braking distance of fourwheelers and six-wheelers and articulated vehicles was aboutp0.3 g. The few eight-wheelers and vehicles towing trailers which were tested had an average braking distance of about 50 per cent. longer, the overall deceleration being only 0.2 g. (67 ft. from 20 m.p.h.).
Hand-brake tests were made at the same time, and 30 per cent, of commercial vehicles could not attain the maximum deceleration of 0.3 g.